When the movement of a fluid is suddenly obstructed, e.g., by valve closure, the kinetic energy of the moving fluid causes the fluid to be compressed in the immediate vicinity of the obstruction. The local expansion of the fluid which follows the maximum compression appears as a reversely directed pressure or shock wave that propagates through the fluid. This phenomenon is commonly referred to as a water hammer, even though carrier fluids other than water can be used to generate the same effect.
In oil and gas well drilling, it is common to use a down hole motor which is driven by a flow of incompressible fluid (preferably high specific gravity fluid drilling mud) which rotates an attached drill bit. The mud can also act to clear cuttings from the hole and provide down hole pressure control (and thereby inhibit blow outs).
However, particularly when drilling in rock and hard materials and when directional (i.e. non-vertical) drilling, there may be insufficient down hole weight on the drill bit to fracture rock and achieve an economically feasible rate of progress. A fluid hammer drill may be used to increase the rate of progress. Water is preferred for the fluid hammer because mud, with its high viscosity, tends to rapidly wear the internal surfaces of the hammer.
It may be preferred to have mud driving the drill bit rotation, and to also have a flow of water or other less dense fluid to drive the fluid hammer. A fluid hammer with impacts in more than one section, and where the impacts can form additive shock waves, is more preferred—including where drilling is done with coil tubing.